1) Field of the Invention
The present invention relates to a field sequential color display device that achieves multicolored display by dividing a field into a plurality of subfields, displaying a different color image in each subfield, and subjecting different color images to color mixture through the action of integration along time-axis at human eyes.
2) Description of the Related Art
Several types of field sequential display devices are known. For example, one type of the field sequential display device (see Japanese Patent Application Laid-Open No. 7-333574) is provided with a light source that emits a light with broadband wavelengths, or a white light; and a disc filter that includes a plurality of filters for respectively transmitting red (R), green (G), and blue (B) lights. When the disc filter is rotated, a wavelength zone for transmitting a light is sequentially switched to another on a subfield basis.
Another type of the field sequential display device (see Japanese Patent Application Laid-Open No. 6-110033) is provided with a color light source that emits red, green, and blue lights; a light source driver that drives the color light source; a shutter that controls the amount of transmission of the light emitted from the color light source based on display information; and a shutter controller that controls the shutter. This display device emits different color lights on a subfield basis and accordingly controls the shutter. The color light source includes a red light emitting diode (hereinafter, “red LED”), a green light emitting diode (hereinafter, “green LED”), and a blue light emitting diode (hereinafter, “blue LED”).
The display device disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 7-333574 employs a stabilized white light source such as a lamp, and filters of R, G, and B for full-color display. Thus, a mechanical component, such as a motor that rotates the filters, is required which results up-sizing and increase in power consumption.
The display device disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 6-110033 better in that it does not require a mechanical component. However, a full-color display is impossible in this color display until the blue LED was not developed. Thus, the major use of this display device is limited in multicolored display of around four colors on a simple guideplate, for example.
Recently, a high-emission blue LED has been developed. Thus, a full-color field sequential display can be developed in combination of the high-emission blue LED with the conventional red LED and green LED. This display device has a wider color reproduction range on a chromaticity diagram of red, green, and blue, and excellent performance of full-color display, in comparison with the display device of the color filter type disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 7-333574. However, color of the light emitted from LEDs varies. For example, the green light emitted from one green LED may be slightly reddish while the green light emitted from another green LED may be slightly bluish. Such color variation may cause subtle color differences from LED to LED even if the colors of the emitted lights are identical. The color variation may occur even in the same LED due to temporal variations of a current for driving the LED and temperature.
FIG. 20 shows the color light emission characteristic of a field sequential display device that employs red, green, and blue LEDs for full-color display. In FIG. 20, vertical axis represents an amount of sub-image data and horizontal axis represents time. Moreover, the color of the light output changes from red to green and then to blue as the time passes. The amount of sub-image data differs from color to color when the shutter operates in accordance with individual color data of red, green, and blue. In FIG. 20, R means a period in which the red LED is ON, G means a period in which the green LED is ON, and B means a period in which the blue LED is ON.
The red LED turns on during R period to display a red image with an amount of transmitted light based on red data Dr (=D1). The green LED turns on during G period to display a green image with an amount of transmitted light based on green data Dg (=D2). The blue LED turns on during B period to display a blue image with an amount of transmitted light based on blue data Db (=D3).
Full-color display with the use of red, green, and blue LEDs can be performed utilizing the action of integration along time-axis at human eyes under field sequential driving as shown in FIG. 20. In FIG. 20, as for parts having an equal data value in the individual color data of red Dr, green Dg and blue Db, that is, the lower parts of Dr, Dg and Db below a chain line denoted with CL, integration of the colors of red, green, and blue results in white light emission. Equivalent white data, Dw, corresponding to the white light emission is subjected to color mixture with parts of the individual color data Dr, Dg and Db above the chain line CL, (Dg′ and Db′ in FIG. 20), to achieve color display.
As the LED that emits white light (hereinafter referred to as white LED), there is publicly known one that includes a blue LED covered with a resin that contains fluorescent particles (see Japanese Patent Application Laid-Open No. 10-65221 and U.S. Pat. No. 6,069,440). There is another publicly known white LED that includes a blue LED covered with a resin that contains fluorescent particles and strontium to compensate for red (see Japanese Patent Application Laid-Open No. 2000-244021).
In the system for full-color display using red LEDs, green LEDs and high-emission blue LEDs, color balance in the white light emission greatly effects on the performance of the full-color display. Nevertheless, there is a problem because of difficulty to keep the chromaticity of the white level unchanged. The reason is given below. As described above, the white light emission is expressed as a mixed color when the LEDs of red, green, and blue are sequentially driven based on the individual color data Dr, Dg and Db.
Due to individual differences such as variations in luminous intensity and variations in forward voltage of individual LEDs, subtle color differences may occur depending on LEDs even if the emission colors are identical as described above. This is specifically shown in the x-y chromaticity diagram of FIG. 21. In FIG. 21, the reference numeral 101 denotes a range of colors present in the world. The reference numeral 102 denotes a range of colors that can be expressed in the National Television System Committee (NTSC) system. The reference numerals 103, 104 and 105 denote ranges of emission colors from LEDs of red, green, and blue, respectively.
In this x-y chromaticity diagram, the emission color range 103 of the red LED has x of about 0.57 to 0.64 and y of about 0.30 to 0.35. The emission color range 104 of the green LED has x of about 0.24 to 0.41 and y of about 0.54 to 0.65. The emission color range 105 of the blue LED has x of about 0.14 to 0.29 and y of about 0.05 to 0.21. Through mixture of the emission colors from LEDs of red, green, and blue that have such the color ranges, a white emission color range 106 can be obtained with x of about 0.30 to 0.50 and y of about 0.21 to 0.46.
Thus, the variations in emission colors of LEDs vary white levels from display device to display device. It is therefore difficult to equalize the chromaticity of white levels in different display devices. In a large display provided with plural LEDs per color, a light emitted from each LED interferes with others. Accordingly, it is extremely difficult to adjust the white level.
Even if differences in chromaticity of the white level among plural displays can be adjusted at the steps of producing the displays, temperature dependence present in a luminous intensity characteristic of each LED causes the following problem. As the temperature elevates, luminous intensities of red, green, and blue reduce at reduction rates each intrinsic to respective emission colors, resulting in a deviation from the initial white level already adjusted. This is specifically shown in the x-y chromaticity diagram of FIG. 22. In FIG. 22, the reference numeral 111 denotes chromaticity of the white level at 25° C., and the reference numerals 112 and 113 denote chromaticity of the white level at −10° C. and 50° C.
According to FIG. 22, x has a value equal to about 0.41 at 25° C. To the contrary, x has a value equal to about 0.50 at −10° C. and about 0.35 at 50° C. In general, if the value of x is deviated about 0.02, the color is identified as a different one. Therefore, the value of x fluctuates within 0.35 to 0.50 due to the temperature characteristic as in the example of FIG. 22, and the color can no longer keep white depending on temperatures. As a measure against this problem, it is considered to provide an additional temperature compensator. Each LED is different from others in power consumption and in amount of heat, however. Accordingly, even if the additional temperature compensator is provided, it is difficult to effectively suppress temperature fluctuations.
It is possible to compensate for fluctuations in environmental temperatures to some extent. The need of individual non-linear compensations for fluctuations and the presence of variations in luminous intensities cause a problem of complex and large temperature compensators.
Each LED is individually different in lifetime and durability and accordingly has a problem that the white level is varied in a long term. This is not disadvantageous, however, if the use is not for a long time.
In the conventional art, LEDs of red, green, and blue are turned on simultaneously to create white. In particular, even if LEDs of red, green, and blue are turned on sequentially to create white using the action of integration along time-axis at human eyes, it is extremely important and difficult to keep the chromaticity of the white level unchanged.